1. Field of the Invention
This invention relates to active matrix subtractive color displays, and more particularly to miniature displays which use liquid crystals as polarization rotation elements and retain light propagation through the display mechanism within projections of the liquid crystal pixels.
2. Description of the Related Art
Active matrix liquid crystal cells that are divided into an array of pixels, with the liquid crystal alignment within each pixel subject to independent electronic control, are well known. A portion of such an active matrix array is illustrated in FIG. 1. A grid of electrical lead lines 2 provide activating signals to control transistors 4 within each pixel 6 of the cell. Transparent plate electrodes (not shown) are provided over the front and rear surfaces of each pixel, and an electric field is established between the electrodes for each pixel in accordance with the signal applied to the pixel transistor 4. The liquid crystals within each pixel assume an angular orientation that varies with the field strength. The liquid crystal orientation in turn establishes a polarization direction for light transmitted through the pixels. The polarization angle of polarized light transmitted through each pixel can thus be controlled by applying a desired pattern of electrical signals to the various pixel transistors.
Active matrixes have been used in the past in subtractive color displays, such as that disclosed in U.S. Pat. No. 4,416,514, issued Nov. 22, 1983. The basic construction of this device is illustrated in the exploded view of FIG. 2 for a single pixel; an array of many such pixels would normally be employed. A light source 8 illuminates a color-neutral linear polarizer plate 10. Next in line is a liquid crystal cell 12 that rotates the polarization angle of the various pixels up to 90.degree.. This is followed in succession by a cyan color linear polarizer 14 with a polarization axis 16 at right angles to the polarization axis 18 of neutral linear polarizer 10, another liquid crystal cell 20 that has an unenergized polarization axis perpendicular to that of cell 12, a magenta color linear polarizer 22 whose polarization axis 24 is perpendicular to that of the cyan polarizer 16, a third liquid crystal cell 26 whose unenergized polarization axis is perpendicular to that of cell 20 and parallel to that of cell 12, and finally a yellow color linear polarizer 28 whose polarization axis 30 is perpendicular to that of polarizers 10 and 22, and parallel to that of polarizer 14.
The liquid crystal cell 12 consists of a liquid crystal layer 32 sandwiched between transparent electrodes 34, 36, with an electrical input at 38 to control the voltage across the electrode plates. Similarly, the liquid crystal cell 20 consists of a liquid crystal layer 40 bounded by transparent electrode plates 42, 44 with an electrical input 46, and cell 26 consists of a liquid crystal layer 48 bounded by transparent electrode plates 50, 52 with an electrical input 54.
In practice, each of the liquid crystal "cells" 12, 20, 26 is a single pixel within a much larger pixel array, with each of the pixels independently controlled. The assembly functions as a subtractive color display by applying electrical signals to each of the cells that cause their liquid crystals to assume desired angular orientations, such as .theta.1, .theta.2 and .theta.3 as illustrated. The light from source 8 which emerges from neutral polarizer 10 is horizontally polarized. Color polarizers 14, 22 and 28 respectively filter out red, green and blue. The amount of filtering varies with the difference between the polarization angle of each polarizer and the polarization angle of the light incident on that polarizer; full filtering is achieved with a 90.degree. difference, while no filtering results from parallel polarization angles. Depending upon the polarization rotation imparted by each of the liquid crystal cells, the viewer sees a gamut of color hue, saturation and brightness.
When used in miniaturized applications, such as helmet mounted displays, there is a significant problem of light loss as the light is processed through the display. For example, a 1,024.times.1,280 pixel display with three color subtractions will have a total of 3.times.1024.times.1280=3,932,160 individually energized liquid crystal pixels. For miniaturized displays, the center-to-center spacing between liquid crystal pixels may be about 25 micrometers (microns) and the total pixel areas may be about 25.times.25 microns, of which the transparent area occupies about 18.times.18 microns. Collimated light from a point source, after passing through the transparent area of each pixel, is diffracted into increasingly greater diverging angles as the size of the pixel is reduced. For an 18.times.18 micron pixel, the light intensity at a distance of 2 mm from the pixel drops to approximately 7% of the intensity at about 0.2 mm. This loss of light is aggravated if the input beam is divergent rather than collimated.
The light spreading has a very detrimental effect for subtractive color displays. Whereas the light that passes through each pixel of the first liquid crystal layer will ideally pass through the corresponding pixels in the subsequent layers, the effect of light spreading is that a portion of the light from a given pixel in the first layer will be transmitted to non-corresponding pixels in subsequent layers, or lost from the display completely if the pixel is in the vicinity of the periphery. This results in a degradation of both resolution and brightness.